The present disclosure relates to a transmission filter capable of high-speed processing for achievement of high-speed communication. Conventionally, in a communication system that transmits a signal from an antenna, a signal with a frequency band necessary for a transmission signal is extracted by a transmission filter, the signal is modulated by mixing with a carrier in a radio frequency (RF) section, and then the resultant signal is transmitted from the antenna as the transmission signal. IEEE 802.15.3c defines a root raised cosine (RRC) filter of a (roll-off factor)=0.25 as a transmission filter for milliwave communication systems. To implement such an RRC filter of α=0.25 in analog form, a 14th or higher order filter is necessary, and one-chip implementation of such a filter is not practical. Thus, implementation of a transmission filter poses a large problem.
To solve the above problem, Japanese Translation of PCT International Application No. 2008-527935 (FIG. 1), for example, proposes a transmission filter implemented using a digital filter and a DAC. The configuration of this transmission filter is shown in FIG. 9.
In FIG. 9, a transmission filter 1 includes a digital filter 2, a digital-to-analog converter (DAC) 3, a PLL circuit 4, and a low pass filter (LPF) 5. In the transmission filter 1, the output of the digital filter 2 is converted to an analog value by the DAC 3, and then a high-frequency component of the analog value is rejected by the LPF 5 connected downstream of the DAC 3. Both the digital filter 2 and the DAC 3 operate with a clock supplied from the PLL circuit 4 via a clock line 6. The transmission filter 1 is used for communication at a speed on the order of several MHz.